Trona deposits in Wyoming consist mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) containing about 3 to 15 percent or, more generally, 5 to 10 percent of insoluble impurities. Principal impurities contained in the trona include silicaceous matter, e.g. quartz, feldspar and clay, as well as carbonaceous matter.
In the production of sodium carbonate from natural trona, it is usual procedure to calcine the trona at elevated temperatures to obtain crude sodium carbonate: EQU 2(Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) .fwdarw. 3Na.sub.2 CO.sub.3 + 5H.sub.2 O + CO.sub.2.
calcination of the trona effects moisture removal, reduction or elimination of carbonaceous matter, as well as transformation of the sodium sesquicarbonate to sodium carbonate. However, calcination at elevated temperature also causes sodium carbonate to react with silicate contained in the trona to form soluble silicates. In order to avoid rendering large proportions of silicate into water soluble form, it has been proposed to calcine trona at relatively low temperatures in the order of about 150.degree. to 200.degree. C. At these low calcination temperatures, however, relatively large proportions of the carbonaceous matter contained in the trona remain in the calcined trona and contaminate process liquors prepared therefrom. Contamination with carbonaceous matter could be held to a minimum or entirely avoided by conducting calcination at higher temperatures. Unfortunately, however, soluble silicate formation increases with increasing calcination temperatures. Hence, usual operation involves a compromise under which both soluble silicates as well as carbonaceous matter will accumulate in the sodium carbonate crystallizer mother liquors prepared from calcined trona. Additionally, such crystallizer mother liquors will accumulate other impurities, such as chlorides and sulfates. Unless steps are taken to reduce contamination of sodium carbonate crystallizer mother liquors with soluble silicates and carbonaceous matter, as well as with chlorides and sulfates, these contaminants may become significant impurities in the soda ash prepared from such liquors. Carbonaceous matter, if not removed from the sodium carbonate crystallizer mother liquors, not only contaminates the soda ash product; but such carbonaceous matter also includes crystal habit modifiers which adversely modify the crystal structure of the sodium carbonate monohydrate produced by crystallization from the sodium such liquors, and further may cause troublesome foaming of such liquors.
In any event, in the process for making sodium carbonate from trona by the steps of crushing the trona, calcining the crushed trona, dissolving the crushed trona in aqueous medium, generally comprising sodium carbonate crystallizer mother liquor, to obtain sodium carbonate solution, crystallizing sodium carbonate monohydrate crystals from that solution (in apparatus herein referred to as sodium carbonate process crystallizers), followed by separating the sodium carbonate monohydrate crystals and dehydrating them, as by heating, to obtain anhydrous soda ash, it is usual practice to purge a small portion of the sodium carbonate crystallizer mother liquor in order to control contaminants, such as soluble silicates, carbonaceous matter, chlorides and sulfates, within tolerable limits. The sodium carbonate crystallizer mother liquor so purged (hereinafter sometimes referred to as sodium carbonate crystallizer purge liquor, crystallizer purge liquor, or simply purge liquor) generally is committed to waste.
It is an object of the present invention to provide a method for recovering soda values from sodium carbonate crystallizer purge liquor from the process of making soda ash from trona. It is another object of the present invention to provide a method for recovering sensible heat from such sodium carbonate crystallizer purge liquor and/or sensible heat and latent heat of vaporization from the sodium carbonate process crystallizer vapors. It is a further object to recover water from the crystallizer purge liquor in the form of sodium carbonate solution.